1. Field of the Invention
The present invention relates to a wafer holding head to be used in an apparatus for polishing the surface of a semiconductor wafer in the process for manufacturing semiconductors and a wafer polishing apparatus, and a method for manufacturing wafers.
The specification of the present invention is based on the Japanese Patent Applications (Japanese Unexamined Patent Application Nos. 11-67583, 11-78688, 11-135017, 11-175950 and 11-251429), and the content of these Japanese applications are incorporated herein by references.
2. Description of the Related Art
Fine patterning of semiconductors have been developed in recent years as a result of development of highly integrated semiconductor devices. Since fine patterning of multilayer structures have been made easy and secure, it is particularly important to planarize the surface of semiconductor wafers to the best in the manufacturing process.
Finer planarization of the surface of the semiconductor wafers allows patterning precision to be improved besides making focusing of the exposed light easy when a photolithographic process is used for patterning. In addition, production of the semiconductor wafers can enjoy a low cost because the work efficiency is improved without providing complicated equipments for manufacturing the semiconductor wafers.
A chemical-mechanical polishing method (a CMP method) has been highlighted for this purpose since the method can polish the surface film with a high degree of planarity.
The surface of wafers are mechanically and chemically polished and planarized using a slurry of an abrasive such as an alkaline slurry containing SiO.sub.2, a neutral slurry containing SeO.sub.2, and an acidic slurry containing Al.sub.2 O.sub.3 (these are simply referred as a slurry hereinafter) in the CMP method. An example of the wafer polishing apparatus used for this method is shown in FIG. 31.
In FIG. 31, the wafer polishing apparatus 200 is provided with a wafer holding head 201 holding a wafer to be polished, and a polishing pad 202 affixed on the entire surface of a disk shaped platen 203. A plurality of the wafer holding heads 201 are attached at the lower portion of a carousel 204 as a head driving mechanism, which is rotatably held on a spindle 211 and rotates with a planetary motion on the polishing pad 202. The center of the platen 203 may be provided eccentric to the center of the revolution of the wafer holding heads 201.
The platen 203 is horizontally placed at the center of a base 205, and is allowed to rotate around its axis line with a platen driving mechanism 206 provided in the base 205. Guide posts 207 are provided at the side portions of the base 205, while an upper mounting plate (a bridge) 209 for supporting a carousel driving mechanism 210 is disposed between the guide posts 207. The carousel driving mechanism 210 serves for allowing the carousel 204 provided below the driving mechanism to rotate around the axis line.
Bridge supports 212 are disposed so as to protrude from the base 205, and gap adjustable gap 213 are provided at the tips of the bridge supports 212. Support plates 214 are disposed in opposed relation to the bridge supports 212. These support plates 214 are fixed to the upper mounting plate 209 and protrude downward from the upper mounting plate 209. The distance from the wafer holding head 201 to the polishing pad 202 are properly adjusted by allowing the bridge supports 212 to contact the support plates 214. Wafers W are polished by allowing the wafers W held by the wafer holding head 201 to contact the surface of the polishing pad 202, followed by allowing the carousel 204 and the platen 203 to rotate.
U.S. Pat. No. 5,205,082 discloses an improved polishing apparatus having a wafer holding head as shown in FIG. 32. The wafer holding apparatus comprises a hollow head body 221, a diaphragm 222 horizontally expanding in the head body 221, and a carrier 224 fixed at the lower face of the diaphragm 222. A retainer ring 232 is disposed at the outer circumference of the carrier 224 in a concentric relation with a slight gap between them, and the retainer ring 232 is also fixed to the diaphragm 222. Further, stoppers 223 of the main head 221 are disposed at the outer circumference of the retainer ring 232 in a concentric relation with a slight gap. The carrier 224 and the retainer ring 232 are supported to the head body 221 in a floating manner as described above. An air chamber 226 is formed at upward of the diaphragm 222 by taking advantage of the head body 221 and the diaphragm 222, wherein a compressed air is supplied to the air chamber from a compressed air source 230 through inside of a shaft 228.
Polishing works are carried out by allowing a wafer W, fixed by being affixed to the carrier 224 via an insert S, to contact a polishing pad 202. The contact pressure is made to be adjustable by changing the pressure of the air supplied to the air chamber 226. The conventional wafer polishing apparatus as described above can even the contact pressure of the wafer W as described above with an advantage for improving uniformity of the polished face of the wafer. The diaphragm 222 to be used herein is usually made of an elastic material such as rubber and an extremely thin plate of a metal, which has a surface rigidity to an extent not to inhibit the movement of the carrier 224 along the axis line direction.
Meanwhile, a frictional force always generates between the wafer W and the polishing pad 202. Consequently, a horizontal force and torque applied to the carrier 224 and the retainer ring 223 are received by the retainer ring 232, the stopper 223 of the head body 221, and the diaphragm 222 in the conventional wafer polishing apparatus as described above. The carrier 224 and the retainer ring 232, and the retainer ring 232 and the stopper 223, each being a circular contour, are fitted with each other with a slight gap as described above. Accordingly, although the horizontal force applied to the carrier 224 and the retainer ring 232 may be received by allowing the side wall face of the former to contact the side wall face of the latter, the torque around the axis line is received only by the diaphragm 222.
The torque applied to the diaphragm 222 during the polishing work is unstable since it is caused by the frictional force, sometimes exceeding the strength limit of the diaphragm 222 to damage the diaphragm 222 made of a thin material. Since the frictional force caused by polishing, and the torque applied to the diaphragm 222 increase in response to the increase of the force for pressing the wafer W to the polishing pad 202 and increase of polishing rate, the pressing force and polishing rate is limited for preventing the diaphragm 222 from being damaged. Therefore, the polishing efficiency (polishing turns per unit time), precision of the polished face, and uniformity limit are determined by the limitations of the polishing conditions as described above.
Although increasing the strength of the diaphragm 222 by using a thicker material may be contemplated as a countermeasure for the foregoing problems, surface rigidity of the diaphragm will be inevitably increased to deteriorate trailing property of the carrier 224 along the axis line direction, thereby adversely affecting planarity and uniformity of the polished face.
When the wafer is polished using the wafer polishing apparatus 200 as hitherto described, whether the polished face of the wafer W has attained a desired conditions (detection of the end point of polishing, or end of the process) or not has been judged by observing, for example, fluctuations of the rotational power of the platen driving mechanism 206. That is, when polishing of the wafer W is insufficient, the frictional force acting between the polishing pad 202 and the wafer W is not stabilized to cause fluctuation, while the frictional force is stabilized when the wafer W is polished to a desired polished face. Since the platen 203 is allowed to rotate at a constant speed, the rotational powder of the platen driving mechanism 206 increases, for example, when the polishing resistance is large, and decreases when the polishing resistance is small. The polished face of the wafer W is judged to be in a desired state when the observed values obtained by observing the rotational power of the platen driving mechanism 206 has been stabilized.
However, the polishing end point (the end of the process) can not be individually detected for the plural wafer holding heads 201 merely by observing the fluctuation of the rotational power of the platen driving mechanism 206, thereby causing the problems of excess polishing and insufficient polishing of the wafer W, or mixing of excessively polished wafers and insufficiently polished wafers.
The platen 203 is often allowed to idle without making the wafer W to contact the polishing pad 202. When the wafer comprises a substance having intrinsically a small frictional resistance, fluctuation of the rotational power of the platen driving mechanism 206 between the states when the wafer W is on the way of polishing and when polishing has been completed becomes so small that the fluctuation can not be distinguished from idling components of the platen 203.
While a method for detecting the rotational power of respective wafer holding heads 201 for detecting the polishing end point may be contemplated, the method has so poor response that it was impossible to accurately detect the force acting on the wafer W. Since the force detected by the method above contains the frictional force acting on the contact portions between the portions of the wafer holding head 201 not holding the wafer W and the polishing pad 202, it was impossible to accurately detect the polishing end point.
The polishing pad 202 is deteriorated by polishing the wafer W. While a dressing treatment is applied to the deteriorated polishing pad 202, it is difficult to judge whether the dressing treatment should be applied or not during polishing of the wafer. Consequently, the timing for practically applying the dressing treatment has been determined to be a prescribed time interval (for example, after polishing a set of wafers), irrespective of the longer polishing time or shorter polishing time. Accordingly, polishing of the wafer is forced to halt for applying the dressing treatment even when the polishing pad 202 has not been deteriorated, thereby resulting in a poor work efficiency.
The polishing resistance acting on the wafer W gradually increases as the polishing pad 202 is deteriorated in some cases. When the difference of the polishing resistance changes between the state during polishing and the state when polishing has been completed is small, for example when the polishing end point is determined by the state when roughness on one layer (for example, an oxide film layer or an insulation layer) has been removed, the change can not be distinguished from the polishing resistance change due to deterioration of the polishing pad 202, making it difficult to detect the accurate polishing end point from fluctuation of the rotational power of the platen driving mechanism 206.
The force detected as described above contains the frictional force acting on the contact portions between the wafer holding head 201 not holding the wafer W, and the polishing pad 202, as well as the frictional force acting on the wafer W. Therefore, it was impossible to accurately detect the polishing end point when the frictional force changes due to deterioration of the polishing pad 202.
FIG. 33 illustrates an enlarged perspective view of the main part of the wafer polishing apparatus in an another example. In the wafer polishing apparatus 241 shown in FIG. 33, a polishing pad 244 comprising, for example, hard polyurethane is provided on a disk shaped rotation table 243 (a platen) attached to the center axis 242, and a wafer holding head 245 capable of revolving is disposed, with its face in opposed relation to the polishing pad 244, at the eccentric position to the center axis 242.
The wafer holding head 245 is approximately formed to be a disk shape having a smaller diameter than that of the polishing pad 244, and the wafer W is held at the lower part, or at the tip, of the head, to allow the wafer to come in contact with the polishing pad 244 while the upper end of the head is supported with an arm (not shown).
A fluidized slurry SL of an abrasive is fed on the polishing pad 244 when the wafer W is polished, and the slurry SL flows between the wafer W held with the wafer holding head 245 and the polishing pad 244. Since the wafer W held with the wafer holding head 245 rotates while the polishing pad 244 simultaneously revolves around the center axis 242, one face of the wafer W is polished with the polishing pad 244.
The slurry SL invades into the wafer holding head in the wafer polishing apparatus using the CMP method. The slurry penetrating into the wafer holding head is dried while polishing is continued, or is degraded by the heat of abrasion generated during polishing, thereby the slurry forms an aggregate or a gel, or is converted into a solid or a semi-solid. When the solid or semi-solid formed as described above flows out from the wafer holding head onto the polishing pad 244 (202), the solid or semisolid causes damages on the surface of the wafer W. Accordingly, a cleaning device such as a shower was used for enabling the wafer holding head to be cleaned, or a wafer holding head 245a capable of cleaning from the inside of the head (see the front cross section in FIG. 34) was used in the related art.
The wafer holding head 245a is provided with a head body 252 comprising a top plate 253 and a cylindrical circumference wall 254 provided below the outer circumference of the top plate 253, a diaphragm 255 vertically expanded to the head axis in the head body 252, a pressure adjusting mechanism 271 for adjusting the pressure (for example, air pressure) in a fluid chamber 264 formed between the diaphragm 255 and the head body 252, a carrier 256 fixed at the diaphragm 255 and provided so as to be able to displace along the direction of the head axis together with the diaphragm 255, and a retainer ring 257, which is provided so as to be able to displace along the head axis direction, provided so as to be positioned between the outer circumference face of the carrier 256 and the inner wall face of the head body 252. The carrier 256 is provided to hold the wafer face to be polished, and the retainer ring 257 is provided to lock the wafer W by contacting the polishing pad 244 (202) during polishing.
A pure water tube 262 as a device for cleaning the wafer holding head 245a is provided in the head body 252. The pure water tube 262 is coupled with a pure water feed tank 272 via a water-supply hose 272a, which leads pure water supplied from the pure water feed tank 272 to the lower part of the head body 252 to allow the water to spout among the head body 252, the carrier 256 and the retainer ring 257. The pressure adjusting mechanism 271 is provided for controlling the pressure, for allowing the wafer W held by the carrier 256 to contact the polishing pad 242 (202), within an appropriate range suitable for polishing, by adjusting the force for displacing the carrier 256, which displaces in response to the pressure in the fluid chamber 264, or together with the diaphragm 255, along the head axis direction.
However, the water supply hose 272a should be detached from the coupling port 262a of the pure water tube 262, in order to allow the wafer holding head 245a to rotate during polishing of the wafer W. Accordingly, the wafer polishing apparatus is forced to be on alert for every cleaning work of the wafer holding head 245a, and currently the water supply hose 272a is manually attached and detached.
Cleaning of the wafer holding head 245a is actually takes much time as described above, forcing the wafer polishing apparatus to be on alert during the cleaning work. Therefore, the wafer holding head 245a has been cleaned after completing the daily polishing work, not to adversely affect the work efficiency of wafer cleaning. However, the foregoing procedure brings about a cleaning work after solids or semi-solids have been formed by aggregation or gelation of the slurry SL. Once such solids or semi-solids have been formed, they can hardly removed by cleaning, and the remaining solids or semi-solids inevitably flow on the polishing pad 242 (202) in the succeeding polishing work.
When the wafer holding head is cleaned with a cleaning apparatus such as a shower, the slurry SL by any means remains in the wafer holding head, because the cleaning operation is applied from the outside of the head.
In addition, the wafer holding head 245a is heated by the heat of friction generated by polishing the wafer W, which adversely affect chemical reactions between the slurry SL and the wafer W to make it difficult to maintain an ideal condition for polishing the wafer W. Also, since the carrier 256, the wafer W and other members are subjected to heat-deformation, it is currently difficult to maintain machining accuracy of the wafer W.